Published at 4:36 PM EDT on Jun 15, 2017

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When Paul McCartney was 16, he wrote "When I'm 64." The song eventually turned up on the Beatles’ "Sgt. Pepper's Lonely Hearts Club Band," a landmark album released 17 days before his 25th birthday.

Early next month, McCartney heads out on a U.S. tour that begins a day after the 60th anniversary of 15-year-old Paul's first meeting 16-year-old John Lennon at a church fair in Liverpool.

McCartney turns 75 Sunday – the latest milestone for a music great who once seemed too young to pen a sentimental tune about aging and now might seem too old to play rocking three-hour shows, night after night.

But McCartney's made a storied career out of defying age and expectations.

The arrival of the Beatles and the mania that ensued displayed the power and possibility of youth in pop culture and beyond. The band's endurance – evidenced recently by record-setting Spotify streams and the re-ascension of "Sgt. Pepper" on the charts – underscores the timelessness of their work and appeal.

That McCartney and Ringo Starr, who turns 77 next month, keep touring and making new albums when they could easily retire to the Isle of Wight, or wherever they want, speaks to the life force that surges through great music.

McCartney and Starr likely get as much out of their performances as their intergenerational legions of fans do. The surviving Beatles also are playing for Lennon and George Harrison, both lost before their time.

Even with the “many years from now” cited in “When I’m 64” long past, McCartney appears destined to keep performing his songs, new and old.

The Beatles exist on a plane where past and present merge in real time, with every discovery and rediscovery of their catalog. Anniversaries and birthdays offer a fine excuse not only to listen, but to celebrate the never-ending age the Beatles and Paul McCartney.